Conventionally, automatic transmission systems are widely used in automotive vehicles so that the shifting operation of the transmission may be hydraulically controlled, and there are those which carry out an transmission operation according to the vehicle speed and the throttle opening serving as a parameter representing the engine load according to prescribed transmission characteristics In such automatic transmission systems, when carrying out a downshift transmission operation from a high speed stage to a low speed stage by depressing an accelerator pedal or by a kick-down operation, the shock due to the abrupt change in the transmitted torque can be mitigated by appropriately increasing the rotational speed of the engine according to the gear ratio of the low speed stage and synchronizing the gears that are about to be meshed with each other.
For instance, as disclosed in Japanese patent publication (kokoku) No. 49-40585 (corresponding to British patent application No. 21353/69 filed in the name of Borg-Warner Limited), there is known the oil ejection control valve (refer to the 2-3 ejection valve 37 in FIG. 5 and the 3-4 ejection valve 38 in FIG. 7) which opens up an oil ejection passage for ejecting oil from an hydraulic engagement element for the high speed stage when the pressure supplied to another hydraulic engagement element for the low speed stage has reached a prescribed level. Japanese patent laid open publication (kokai) No. 61-84450 teaches a similar ejection valve which is placed in an oil ejection passage for an hydraulic engagement element for a high speed stage and is provided with a valve member which is displaced so as to open the oil ejection passage either by the supply pressure of a hydraulic engagement element for a low speed stage or the supply pressure of a hydraulic engagement element for an intermediate speed stage so that the reduction of the transmission shock may be effected even when the transmission gear unit is downshifted by two speed stages at a time, for instance from a fourth speed stage to a second speed stage.
When a kick-down operation is to be made in such an automatic transmission system to shift the speed stage from the high speed stage which, for instance, consists of a fourth speed stage to the low speed stage which, for instance, consists of a second speed stage, the engagement pressure P4 of the fourth speed clutch serving as the hydraulic engagement element for the high speed stage changes as indicated by the imaginary line in the upper part of FIG. 3. In this case, the initial decrease in the engagement pressure P4 is brought about by a narrowed passage such as a fixed orifice provided in an oil ejection passage of the fourth speed clutch, and progresses in a relatively gradual manner. The subsequent reduction in the engagement pressure is also carried out in a gradual manner because the ejection of the pressure of an accumulator associated with the fourth speed clutch for mitigating the shock of clutch engagement proceeds at the same time. As the valve member which has been biased by a spring in the direction to close the oil ejection passage is displaced with the increase in the engagement pressure P2 of the second speed clutch which acts upon a land of the valve member against this spring force, the oil ejection control valve opens up (as indicated by P in the drawing). In other words, since the ejection of the oil is carried out by the oil ejection control valve in addition to the fixed orifice once the point P is reached, the flow resistance in the oil ejection passage is substantially reduced, and the engagement pressure P4 of the fourth speed clutch is relatively rapidly reduced from the time of opening the valve before the engagement pressure P4 is completely removed.
The engagement pressures P2 and P4 of the two clutches are supplied and ejected as described above, and the rotational speed of the engine also changes accordingly as indicated by the imaginary line in the middle part of FIG. 3. The increase in the rotational speed of the engine is gradual in the initial part of the transmission operation, and this is due to the fact that the ejection of pressure from the fourth speed clutch occurs in a gradual manner and the engagement condition of the fourth speed stage allows torque to be transmitted to a certain extent. Thereafter, since the engagement force declines as pressure is removed from the fourth speed clutch, and the engagement pressure of the fourth speed clutch sharply declines by the opening of the oil ejection control valve, the rotational speed of the engine rises sharply. The acceleration of the vehicle body G also changes from a negative value to a positive value as indicated by the imaginary line in the lower part of the drawing and the resulting transmission shock is transmitted to the vehicle occupant because the decline in the engagement pressure of the fourth speed clutch and the rise in the engagement pressure of the second speed clutch are both sharp or abrupt.
In such an automatic transmission system, a similar problem exists when an upshift operation is made from a low speed stage such as a second speed stage to a high speed stage such as a third speed stage. When an oil ejection control valve placed in an oil ejection passage of the hydraulic engagement element for the low speed stage is activated and the oil pressure of the hydraulic engagement element for the low speed stage is ejected, since the engagement pressure for the clutch which constitutes the hydraulic engagement element for the low speed stage or a second speed stage makes a sudden change, as illustrated in the conventional example indicated by the imaginary line in the upper part of FIG. 5, at the same time as the engagement pressure of the hydraulic engagement element for the high speed stage or a third speed stage reaches a prescribed level and the oil ejection control valve is activated (as indicated by P in the drawing), a relatively large change occurs in the vehicle acceleration and the slip ratio in the hydraulic engagement element for the high speed stage as indicated by the imaginary lines in the middle and lower parts of FIG. 5, and this produces a transmission shock.
Since the property of the oil ejection control valve is normally designed so as to minimize the transmission shock when an upshift is made under a load condition in which the accelerator pedal is depressed to a certain extent (power-on upshift), this transmission shock tends to be pronounced when an upshift is made in a light load condition in which the accelerator pedal is substantially released and a relatively large change in the transmission torque occurs because an engine drive situation changes into an engine brake situation by the upshift operation.
FIG. 7 shows an exemplary oil ejection control valve 49' which is employed at the time of upshifting from a second speed stage to a third speed stage. This upshift operation is carried out by supplying oil pressure from a conduit 59 to a third speed clutch 17 via a port 17a leading to an oil chamber of this clutch, and ejecting oil from an oil chamber of a second speed clutch 16 via a port 16a. The port 16a communicates with a fixed orifice 60 via a conduit 52 when a 2-3 shift valve 58 is opened as a result of the upshift operation. The oil ejection control valve 49' is provided with a spool valve member 50' which is normally biased by a spring 57' to a position to close the communication between an inlet port 55' connected to the conduit 52 and an outlet port 56' connected to a pressure sink or an oil reservoir. However, the pressure supplied to the conduit 59 acts upon the spool valve member 50' and urges it to a position to establish a communication between the inlet port 55' and the outlet port 56'.
Therefore, according to this hydraulic circuit, before the engagement pressure for engaging the third speed clutch 17 builds up to a certain level, the oil from the oil chamber of the second speed clutch 16 is ejected solely through the fixed orifice 60, and the decrease in the ejection pressure of the second speed clutch 16 is gradual. However, once the engagement pressure for the third speed stage 17 has built up to a certain level, the oil ejection control valve 49' is opened up, and the ejection of oil from the oil chamber of the second speed clutch 16 takes place in an accelerated fashion.
Thus, according to the prior art oil ejection control valve, the disengagement of the second speed clutch 16 is initially gradual, and is then accelerated in a later stage of the upshift operation so that a substantially transmission shock was inevitable. In particular, when the oil ejection control valve is so designed to minimize the transmission shock under a relatively heavy load condition as is usually the case, a significant transmission shock occurs when an upshift operation is made under a light load condition.